1. Field of the Invention
The present invention relates in general to a surface roughness measuring apparatus, and more particularly to an optical apparatus which utilizes optical interference of two linearly polarized beams for measuring the roughness of a surface of a subject.
2. Discussion of the Related Art
In one type of a known optical surface roughness measuring apparatus, a surface of the subject to be measured is irradiated with two linearly polarized beams of light which have orthogonal planes of polarization and different frequencies. Described more particularly, one of the two linearly polarized beams is converged at an extremely small area of reading spot on the subject surface, while the other linearly polarized beam is converted into parallel rays of light with which a relatively large area including the reading spot on the subject surface is irradiated. The above-indicated one linearly polarized converged beam reflected by the extremely small reading spot serves as a measuring beam, while the other linearly polarized parallel beam reflected by the large area serves as a reference beam. These measuring and reference beams reflected by the subject surface interfere with each other to thereby produce a measuring beat signal which corresponds to the roughness of the subject surface. The known optical surface roughness measuring apparatus is adapted to measure the roughness of the surface of the subject, based on a phase shift or a frequency shift of the measuring beat signal. An example of the above-type of the surface roughness measuring apparatus is disclosed in U.S. Pat. No. 4,905,311 owned by the assignee of the present application. The apparatus disclosed in this patent was developed by the present assignee, but was not known at the time the present invention was made.
In the optical surface roughness measuring apparatus of the type described above, one of the two linearly polarized beams serving as the measuring beam is converged at the extremely small reading spot on the subject surface, while the other linearly polarized beam serving as the reference beam is converted into parallel rays of light for irradiating the comparatively large area around the reading spot. In operation, as the subject is moved relative to the measuring apparatus in a direction perpendicular to the direction of incidence of the measuring and reference beams, the length of an optical path of the measuring beam varies depending on degrees of minute projections and indentations of the surface of the subject, thereby causing changes in the phase or frequency of the measuring beam. On the other hand, the reference beam in the form of parallel rays applied to the comparatively large area of the subject surface does not undergo such a phase or frequency shift due to the roughness of the subject surface, since the amounts of influence of the projections and indentations in the large area on the reflected reference beam are averaged or counterbalanced with each other.
By combining the reflected measuring and reference beams, there is obtained a measuring beat signal having a beat frequency, which varies with the phase or frequency shift of the measuring beam due to the roughness of the subject surface. Thus, the optical surface roughness measuring apparatus is capable of measuring the surface roughness of the subject based on changes in the beat frequency of the measuring beat signal. It is to be noted that since the measuring and reference beams irradiating the subject surface are subject to vibrations or other disturbances during the movement of the subject, the influences of these vibrations or disturbances on the reflected measuring and reference beams are counterbalanced by each other, whereby the phase or frequency of the measuring beat signal is not affected or changed by these disturbances.
Referring to FIG. 3, an example of the above type of the optical surface roughness measuring device will be described in detail. In the figure, reference numeral 30 denotes a laser device in the form of a Zeeman-type laser source, which produces laser light L including two linearly polarized laser beams having orthogonal planes of polarization and slightly different frequencies. The two laser beams may consist of, for example, a P-type linearly polarized laser beam Lp having a frequency fp, and an S-type linearly polarized laser beam Ls having a frequency fs. The laser light L produced by the laser source 30 is split by a non-polarizing beam splitter 32. Namely, a half component of the laser light L is reflected by the non-polarizing beam splitter 32, and received and detected by a reference photosensor 34. This reference photosensor 34 produces a reference beat signal F.sub.B having a beat frequency f.sub.B (=.vertline.fs--fp.vertline.) of the frequencies of the received P-type and S-type laser beams.
The other component of the laser light L is transmitted through the non-polarizing beam splitter 34, and split by a polarizing beam splitter 36 into the P-type linearly polarized laser beam Lp and the S-type linearly polarized laser beam Ls. Described more specifically, the P-type polarized laser beam Lp is transmitted through the polarizing beam splitter 36, and is incident upon a convex lens 38. While the P-type polarized laser beam Lp passes through the convex lens 38 and another convex lens 40, the diameter of the laser beam Lp is increased. That is, a beam expander constituted by these convex lenses 38, 40 converts the P-type laser beam Lp into parallel rays of light having a circular cross sectional shape having an increased diameter. The P-type laser beam Lp transmitted through the lens 38 is reflected by a mirror 42 which is interposed between the two convex lenses 38, 40 so that the laser beam Lp reflected by the mirror 42 is propagated along the optical axis of an objective lens 44 which will be described.
On the other hand, the S-type linearly polarized laser beam Ls is reflected by the polarizing beam splitter 36, and then reflected by a mirror 46. While the S-type laser beam Ls reflected by the mirror 46 passes through a beam expander consisting of two convex lenses 48, 50, the diameter of the laser beam Ls is increased. Namely, the S-type laser beam Ls is converted by the beam expander into parallel rays of light having a circular cross sectional shape. The S-type laser beam Ls from the lens 48 is reflected by a polarizing beam splitter 52 which is interposed between the two convex lenses 48, 50 so that the laser beam Ls reflected by the beam splitter 52 is propagated along the optical axis of the objective lens 44. Thus, the parallel rays of the S-type laser beam Ls are propagated along the same optical axis as the parallel rays of the P-type laser beam Lp. The convex lenses 38, 40, 48, 50 are adapted such that the diameter of the S-type laser beam Ls transmitted through the convex lenses 48, 50 is smaller than that of the P-type laser beam Lp transmitted through the convex lenses 38, 40. The parallel P-type laser beam Lp transmitted through the convex lenses 38, 40 is then transmitted through the polarizing beam splitter 52, and is converged by the convex lens 50 at a given point on the optical axis of the objective lens 44.
The objective lens 44 is positioned such that its front focal point coincides with a rear focal point of the convex lens 50. Accordingly, the P-type linearly polarized laser beam Lp converged by the convex lens 50 is again converted into parallel rays by the objective lens 44 so that a comparatively large area on a surface 18 of a subject 12 is irradiated with the parallel rays of the P-type laser beam Lp. On the other hand, the S-type linearly polarized laser beam Ls which has been converted into parallel rays by the convex lenses 48, 50 is converged by the objective lens 44 at an extremely small reading spot on the surface 18 of the subject 12.
The subject 12 is mounted on an X-Y table 60 which is moved by a drive device 58 in two mutually perpendicular directions in an X-Y coordinate plane perpendicular to the optical axis of the objective lens 44. With the subject 12 being moved with the X-Y table 60, the frequency of the S-type linearly polarized laser beam Ls converged at each small reading spot on the surface 18 is subjected to a Doppler shift .DELTA.fs which corresponds to a change in the height or level of the reading spot on the surface 18 of the subject 12, and a Doppler shift .DELTA.fd caused by vibrations of the subject 12 and other disturbances during the X-Y movements of the X-Y table 60. Namely, the frequency of the reflected S-type laser beam Ls is equal to (fs+.DELTA.fd+.DELTA.fs). Since the P-type linearly polarized laser beam Lp in the form of parallel rays is applied to the comparatively large area on the surface 18 of the subject 12, the amounts of influence of the minute projections and indentations in that area of the surface 18 on the frequency of the P-type laser beam Lp are averaged or counterbalanced with each other. Therefore, the frequency of the P-type laser beam Lp reflected by the surface 18 is subject to substantially no influence of the roughness of the surface 18 of the subject 12, and is influenced only by the disturbances which occur during the X-Y movements of the X-Y table 60. That is, the frequency of the reflected P-type laser beam Lp includes only a Doppler shift .DELTA.fd due to the disturbances, and is equal to (fp+.DELTA.fd). It will be understood that the S-type linearly polarized laser beam Ls irradiating the reading spot serves as a measuring beam, while the P-type linearly polarized laser beam Lp irradiating the relatively larea area on the surface 18 serves as a reference beam.
The P-type and S-type linearly polarized laser beams Lp, Ls which are reflected by the surface 18 of the subject 12 are propagated back to the polarizing beam splitter 36, in the reverse direction along the optical paths along which the beams Lp, Ls are incident upon the surface 18 as described above. The P-type and S-type laser beams Lp, Ls combined with each other by the beam splitter 36 are reflected by the non-polarizing beam splitter 32, and are received and detected by a measuring photosensor 62. This measuring photosensor 62 produces a measuring beat signal F.sub.D which corresponds to a beat caused by optical interference between the P-type and S-type polarized laser beams Lp, Ls. The measuring beat signal F.sub.D has a beat frequency f.sub.D, which is equal to .vertline.(fs+.DELTA.fd+.DELTA.fs)-(fp+.DELTA.fd).vertline.=.vertline.fs-f p+.DELTA.fs.vertline., with the Doppler shifts .DELTA.fd of the two beams Lp, Ls being counterbalanced with each other.
The measuring beat signal F.sub.D and the reference beat signal F.sub.B indicated above are applied to a detecting circuit 64. The detecting circuit 64 produces a beat signal indicative of the Doppler shift .DELTA.fs due to the roughness of the surface 18 of the subject 12, which is obtained by subtracting the frequency f.sub.B (=.vertline.fs-fp.vertline.) of the reference beat signal F.sub.B from the frequency f.sub.D (=.vertline.fs-fp+.DELTA.fs.vertline.) of the measuring beat signal F.sub.D. The beat signal indicative of the Doppler shift .DELTA.fs is applied to a control device 66 which is principally constituted by a microcomputer. The control device 66 is adapted to control the drive device 58 so as to successively move the X-Y table 60 in the X-axis and Y-axis directions, and calculate an amount of displacement Zs in the Z-axis or vertical direction at each reading spot on the surface 18 of the subject 12 during the X-Y movement of the X-Y table 60 (subject 12), according to the following equation (1), in response to the beat signal (.DELTA.fs) received from the detecting circuit 64. Based on the calculated amounts of vertical displacement Zs at the individual reading spots on the subject surface 18, the control device 66 commands a display 68 to provide a three-dimensional indication of the roughness of the surface 18 of the subject 12. EQU Zs=(.lambda./2).intg..DELTA.fs dt (1)
In the optical surface roughness apparatus as described above, the laser light L emitted by the laser source 30 is split by the polarizing beam splitter 36 into the S-type linearly polarized laser beam Ls and the P-type linearly polarized laser beam Lp, which function as the measuring beam and the reference beam, respectively. The measuring and reference laser beams Ls, Lp are propagated along respective optical paths until these laser beams Ls, Lp are incident upon the polarizing beam splitter 52 which guides the laser beams Ls, Lp along the same optical axis. Thereafter, the measuring and reference laser beams Ls, Lp are converted by the convex lens 50 and the objective lens 521 44 into the converged beam and parallel beam, respectively, which irradiate the comparatively small reading spot on the subject surface 18, and the comparatively large area aligned with the reading spot. Since the measuring beam Ls is incident upon the polarizing beam splitter 52 such that the optical axis of the measuring beam Ls is inclined by a given angle with respect to a reflecting surface of the beam splitter 52, an extinction ratio of the measuring beam Ls reflected by the polarizing beam splitter 52 is lowered, resulting in reduction in the signal component of the output of the measuring apparatus. Consequently, the surface roughness measuring apparatus suffers from an unfavorably lowered signal-to-noise (S/N) ratio. The lowering of the S/N ratio is also caused by noises which are included in the measuring and reference laser beams Ls, Lp due to streams of the atmosphere which occur differently in the above two optical paths of these laser beams Ls, Lp. Thus, the surface roughness measuring apparatus of FIG. 3 is not satisfactory in its measuring accuracy because of the low S/N ratio.